1. Field of the Invention
The present invention relates to a method for diffusion detection by means of magnetic resonance (MR).
2. Description of the Prior Art
It was suggested by Stejskal/Tanner in The Journal of Chemical Physics, Vol. 42, Nr. 1, 288-292, 1. Jan. 1965 to execute a diffusion measurement with a spin echo method gradient pulses of the same polarity produced before and after a 180.degree. radio-frequency pulse. In the following, such gradient pulses, which make a nuclear resonance signal sensitive to diffusion, are termed "diffusion gradients" for short. In the original suggestion a spatial resolution was still not carried out in the MR experiment. The method according to Stejskal/Tanner was later applied in connection with imaging MR as well; i.e. in addition to the diffusion gradients according to Stejskal/Tanner, gradients for spatial encoding of the obtained nuclear resonance signal were also generated. In the pulse sequences known hitherto the application of diffusion gradients frequently leads to problems with the imaging, however. For example, in the echo-planar method (EPI) distortions appear on the order of magnitude of 10% of the image size. The effect of fat saturation is extremely compromised.
From the article "Microcirculation and diffusion studies in humans using FT velocity distribution, line scans and echo planar imaging" by D. Feinberg and P. Jakob in SMR Workshop on Future Directions in MRI, 7-8 June 1990, Bethesda, MD, a pulse sequence for diffusion measurement is known wherein two 180.degree. refocusing pulses follow a 90.degree. exciting pulse. In an exemplary embodiment a first gradient pulse is actuated between the 90.degree.exciting pulse and the first 180.degree. refocusing pulse, a bipolar gradient pulse is actuated between the two 180.degree. refocusing pulses, and another gradient pulse is actuated between the second 180.degree. refocusing pulse and the readout time window. The gradient pulses are dimensioned such that they produce a sensitivity to diffusion. Considered over all of the gradient pulses, the gradient time integral is zero, so that stationary spins are not dephased.
Such pulse sequences exhibit the problem that due to the 180.degree. refocusing pulses stimulated echos are generated which interfere or superimpose with other echos, thus leading to image artefacts.